Settling behaviour of Rhopalosiphum padi (L.) in relation to cyanogenic glycosides and gramine contents in barley

In order to examine whether the content of cyanogenic glycosides or gramine in barley affects the host plant preference of the aphid Rhopalosiphum padi (L.) two-choice settling tests were performed with R. padi on different barley varieties with varying amounts of these compounds. The total concentration of cyanogenic glycosides in barley shoots, as determined in 59 varieties, varied between 20 and 1200 nmol (g fw)(-1). Of the 59 varieties, sis with high [ > 700 nmol (g fw)(-1)], six with intermediate [450-650 nmol (g fw)(-1)] and six with low [<200 nmol (g fw)(-1)] content of cyanogenic glycosides were chosen for preference tests with R. padi. The content of gramine was determined in these 18 chosen varieties and was generally either low (close to 0) or high [about 2 mu mol (g fw)(-1)]. There was no correlation between cyanogenic glycoside and gramine content. The aphids did not discriminate between varieties with high, intermediate or low content of cyanogenic glycosides in combinations with high or low gramine content. The results indicate that the concentration of neither of these compounds affected the settling preference of R. padi.